Selective plant breeding has been used to genetically improve crop plants throughout human history. Early hunter-gatherers selectively propagated plants with preferred properties, while early agriculturists deliberately saved seeds from preferred plant types and thereby gradually domesticated a majority of the crop plants known today. Over the past 50 years the combined efforts of plant breeders to successfully develop new crop cultivars have provided the basis for the consistent supply of food in a changing global environment and ever-changing pest and disease populations. This has been a major contributing factor toward the alleviation of world hunger and suffering, and, in some instances, the consequent maintenance of political stability.
The development of plant molecular genetics has facilitated plant breeding methods through such techniques as marker-assisted selection, in which genetic maps of polymorphic markers are used to monitor the selection of plant lines containing desirable alleles of closely-linked genes. Nevertheless, such breeding techniques are ultimately limited by the diversity of the existing genetic material in crop plants. This limitation to the development of crop plants with desirable new genetic traits is substantial in view of the limitations inherent in the genetic diversity of any individual plant species adapted to a select environment in general and the history of inbreeding of crop plants in particular.
Recently developed method of plant genetic engineering offer a means to overcome this limitation by the introduction of new genes into single plant cells from which complete plants can be regenerated via cell and tissue culture methodologies. Genetic engineering of plants has been utilized to improve the quality of crop plant products, such as in the development of an improved tomato with superior ripening characteristics by the expression of an antisense polygalacturonase gene (see Kramer et al. (1994) Euphytica 79: 293-7). Indeed, entire biosynthetic pathways have been altered by plant genetic engineering techniques. For example, starch biosynthesis has been successfully manipulated in tomato (for paste production) and potato (for processing quality and reduced oil uptake) be expression of a bacterial ADP glucose pyrophosphorylase that is insensitive to feedback regulation (see Stark et al. (1996) Ann NY Acad Sci 792: 26-36). There is also great economic potential in the use of transgenic plants engineered for the production of biopharmaceutical compounds. Among the products that are likely to be produced in transgenic plants are cytokines, hormones, monoclonal antibodies, enzymes, and vaccines. Some of these products may be expressed either from stably transformed plants, or from transient expression systems in the form of recombinant plant viral vectors.
The ability to genetically manipulate plants may further allow crops to be grown under conditions of environmental stress or in the presence of plant pathogens. Plants are susceptible to infection by many parasitic, viral, fungal and bacterial organisms, which infect following contact with root, stem, leaf or other plant tissues. Other environmental insults, such as flooding, can damage plants by causing root anoxia. Indeed many agriculturally important crops are destroyed as a result of both infection by pathogens and root damage caused by flooding. For example, corn is highly susceptible to flooding and water logged soil can account for 20-30% losses in the production of this crop in clay-rich soils. The water logging of plant roots causes root anoxia, resulting in a build-up of ethanol and resultant loss of plant viability.
Another environmental stress which seriously affects the productivity of crop plants is salt-stress. Although plant species differ in their relative sensitivity to salt, crop plants are predominantly sensitive to the presence of high concentrations of salts in the soil. Salinity affects more than 40 percent of the world's irrigated lands, including the most productive agricultural areas of the Mediterranean basin, California and southern Asia, where use of poor quality irrigation water has led to the progressive concentration of salts in the soil (Flowers and Yeo (1995) Aust J Plant Physiol 22: 875-884). Approximately 10 million hectares of irrigated land are thought to be rendered useless for crop plant production each year because of the adverse effects of secondary salinization (Szaboles (1987)Acta Agronomica Hungarica 36: 159-72). One strategy for dealing with this problem may be the development of salt-resistant crops, however salt-tolerance does not appear to depend on a single identifiable trait, and traditional plant breeding and the transfer of single traits has been shown to improve salt-tolerance only marginally (Delauney and Verma (1993) Plant J 4: 215-23).
Yet another environmental stress which affects the worldwide productivity of crop plants is exposure to fungal, bacterial and viral pathogens. Certain marine plants may avoid infection by such pathogens, despite continuous exposure to these agents in their aqueous marine environment, by virtue of their production of “anti-fouling” compounds. Fouling is a general term describing the interaction and attachment of various organisms, including marine bacteria and barnacles, to a plant or other surface. The marine seagrass Zostera marina has desirable antifouling characteristics which make it resistant to the attachment of such pathogens and parasites. Zostera marina produces a variety of phenolic acids, including p-(sulfooxy)-cinnamic acid, as natural products. It has been proposed that such phenolic acids confer resistance to so-called wasting disease and inhibit ampthipod grazing, microbial growth and the attachment of marine bacteria, diatoms, barnacles and polychaetes to artificial surfaces. The sulfated phenolic acids have been shown to possess particularly effective antifouling characteristics in laboratory studies (Todd et al. (1993) Phytochemistry 34: 401-4). Significantly, the attachment of pathogenic bacteria, fungi and viruses is the first step toward infection and so these antifouling characteristics are particularly desirable because they preclude infection.